The State Department has issued a worldwide travel alert during the biggest travel season of the year due to "increased terrorist threats."

The alert warns travelers to use caution when in public spaces and using public transportation.

It highlights threats from several groups including ISIS and Boko Haram. It says, "These attacks may employ a wide variety of tactics, using conventional and non-conventional weapons and targeting both official and private interests."

This type of alert from the U.S. is not uncommon. The State Department has issued four worldwide travel alerts in the last four years.